Process and system for recovering white carbon black and high-purity calcium fluoride by using electrolytic aluminum overhaul slag

Abstract

The invention discloses a process and system for recovering white carbon black and high-purity calcium fluoride by using electrolytic aluminum overhaul slag, electrolytic aluminum overhaul slag particles and sulfuric acid are subjected to heating acidolysis in a closed reaction kettle, and mixed gas generated by acidolysis is absorbed by a gas absorption tower to form a mixed solution containing fluosilicic acid and hydrofluoric acid; the mixed solution enters an ammoniation reaction tank for ammoniation reaction, an ammoniation reaction product is subjected to precipitation treatment in the ammoniation reaction tank, bottom layer precipitation is subjected to dehydration, drying and calcination in sequence to form a white carbon black product, and supernate enters a crystallization fluidized bed reactor to be subjected to induced crystallization reaction with a lime solution; dehydrating, washing and drying the generated calcium fluoride crystals in sequence to obtain high-purity calcium fluoride crystals, enabling ammonia gas generated by the reaction to enter an ammonia gas absorption tower to be absorbed to form ammonia water, performing solid-liquid separation on acidolysis residues, performing harmless treatment on solids, performing acid-base neutralization treatment on liquid, and performing acid-base neutralization treatment on the liquid; according to the invention, high-valued recovery of fluorine and silicon resources can be carried out on the electrolytic aluminum overhaul slag.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, and in particular to a process and system for recovering silica and high-purity calcium fluoride from the slag of electrolytic aluminum overhaul. Background Technology

[0002] In the continuous production process of the electrolytic aluminum industry, electrolytic cells, due to long-term exposure to the corrosive effects of high-temperature electrolytic reactions, require regular overhauls, which generate a large amount of electrolytic aluminum overhaul slag. Currently, the industry's main technologies for treating electrolytic aluminum overhaul slag include landfilling, high-temperature incineration, and wet treatment. Each of these technologies has significant limitations:

[0003] Although landfilling is simple to operate and has low initial investment costs, it requires a large amount of land resources and cannot prevent the migration and diffusion of fluorides, posing a long-term potential risk of groundwater and soil pollution, and also causing a direct waste of valuable resources such as fluorine and silicon.

[0004] High-temperature incineration can decompose some organic impurities through a high-temperature environment of 800-1200℃, but this process requires a lot of energy, resulting in high operating costs. In addition, fluorides will be converted into toxic gases such as hydrogen fluoride and volatilize during the incineration process.

[0005] Wet treatment is a widely used technical approach with resource recovery capabilities. Its core principle is to utilize the water solubility or acid solubility of some components in overhaul slag, and through chemical reactions such as leaching, conversion, and precipitation, add chemical agents such as calcium salts and hypochlorite to fix or convert harmful components into harmless substances. Among them, fluoride is mainly converted into water-insoluble calcium fluoride precipitate for recovery.

[0006] However, this method has obvious technical bottlenecks: the method transfers fluorine to the liquid phase, but the aluminum, iron and other impurities remaining in the liquid phase system are easy to form co-precipitates with calcium fluoride, making it difficult to improve the purity of the product and failing to meet the quality requirements of high-end industrial fields for calcium fluoride; on the other hand, the selective separation effect of fluorine and silicon resources is not good, which also leads to a low resource recovery rate.

[0007] While wet processing is the mainstream technology, it faces challenges such as susceptibility to impurities like aluminum and iron during fluoride recovery, leading to difficulties in improving product purity and poor fluorine resource recovery. Simultaneously, silicon resources are not fully recovered and utilized, resulting in waste. In summary, the core technological pain points remain unresolved: either impurities are not effectively separated from the target product, resulting in insufficient purity of the recovered product; or the focus is solely on single resource recovery, neglecting the synergistic utilization of multiple components such as silicon and fluorine, resulting in a low overall resource utilization level. These technological deficiencies make it difficult for existing processes to simultaneously meet the dual requirements of environmental compliance and efficient resource recovery. Summary of the Invention

[0008] To overcome the above-mentioned defects, the present invention provides a process and system for recovering silica and high-purity calcium fluoride from electrolytic aluminum overhaul slag. This process and system can recover high-purity silica and calcium fluoride products respectively, realizing the high-value recovery of fluorine and silicon resources.

[0009] The technical solution adopted by this invention to solve its technical problem is a process for recovering silica and high-purity calcium fluoride from electrolytic aluminum overhaul slag, comprising the following steps:

[0010] Step 1: Pretreatment: Crush and screen the electrolytic aluminum overhaul slag to obtain overhaul slag particles with a particle size of 10-50 mesh;

[0011] Step 2: Sulfuric acid acidolysis: The pretreated overhaul residue particles are added to a closed reactor, and concentrated sulfuric acid with a concentration of 92%-98% is added. The amount of sulfuric acid is set according to the solid-liquid ratio of 1:0.3-1:0.4. The reaction temperature is controlled at 200-350℃. The overhaul residue particles undergo sulfuric acid acidolysis in the closed reactor. The fluorine, silicon, aluminum and iron components in the overhaul residue are converted into hydrogen fluoride, silicon tetrafluoride, aluminum sulfate and iron sulfate respectively under high temperature conditions.

[0012] Step 3: Gas collection: The mixed gas of hydrogen fluoride (HF) and silicon tetrafluoride (SiF4) produced by sulfuric acid acidolysis is transported to the gas absorption tower by the negative pressure of the induced draft fan for collection. The hydrogen fluoride and silicon tetrafluoride gas dissolve in the circulating absorption liquid in the gas absorption tower to form a mixed solution containing fluorosilicic acid and hydrofluoric acid.

[0013] Step 4: Ammoniation for Silicon Removal: The mixed solution of fluorosilicic acid and hydrofluoric acid formed at the bottom of the gas absorption tower is sent to the ammoniation reaction tank. Simultaneously, ammonia water with a concentration of 15%-20% is added to the ammoniation reaction tank to carry out the ammoniation reaction. During the reaction, the pH in the ammoniation reaction tank is controlled at 6.5-7.5. Fluorosilicic acid reacts with ammonia water to form silicon dioxide. The chemical reaction equation is as follows:

[0014] H2SiF6+6NH3·H2O→SiO2+6NH4F+4H2O;

[0015] HF + NH3·H2O → NH4F + H2O;

[0016] Step 5: Silica separation: The mixture in the ammoniation reaction tank is sent to the ammoniation precipitation tank. The silica formed by the ammoniation reaction is separated from the water through precipitation. Then, the solid-liquid mixture at the bottom of the ammoniation precipitation tank is dehydrated.

[0017] Step Six: Drying: The solid filter residue after dehydration in Step Five is dried to form a loose precursor.

[0018] Step 7: Calcination: The precursor obtained in Step 6 is calcined at 500–700℃ for 2–4 hours to completely decompose the ammonium salt. After calcination, the silica product is obtained with a purity of over 99.0%.

[0019] Step 8: Lime-induced crystallization for fluoride removal: The supernatant from the ammoniation sedimentation tank (mainly containing ammonium fluoride) is transported to the crystallization fluidized bed reactor, and a lime solution with a concentration of 2%-5% is simultaneously added to the reactor. The calcium-fluoride molar ratio is controlled between 0.45 and 0.50. Calcium and fluoride combine to form calcium fluoride, as shown in the following chemical equation:

[0020] 2NH4F+Ca(OH)2→CaF2↓+2NH3·H2O;

[0021] Step 9: Crystal discharge and dehydration: The calcium fluoride crystals discharged from the crystallization fluidized bed reactor are periodically discharged. The calcium fluoride crystals discharged from the crystallization fluidized bed reactor are dehydrated, and then the dehydrated calcium fluoride crystals are washed and dried to finally obtain high-purity calcium fluoride crystals.

[0022] Step 10: Ammonia Water Recovery: The gas generated during the defluorination reaction in the crystallization fluidized bed reactor is introduced into the ammonia absorption tower through an induced draft fan and gas absorption pipeline. The gas and the aqueous solution in the ammonia absorption tower come into gas-liquid contact on the surface of the packing material in the absorption tower. The ammonia in the gas is absorbed by the aqueous solution to form ammonia water. The tail gas after ammonia removal is discharged from the top of the ammonia absorption tower.

[0023] Step 11: Harmless treatment of acid hydrolysis residue: After the residual mixture of the overhaul slag is acid hydrolyzed with sulfuric acid, it is separated into solid and liquid. The solid is then treated to be harmless, and the liquid is neutralized with acid and alkali to meet the standards.

[0024] As a further improvement of the present invention, in step two, the closed reactor is heated by electric heating or heat transfer oil heating, the sulfuric acid acidolysis reaction time is 0.5-2h, the mixture is stirred by a stirrer during the sulfuric acid acidolysis reaction, the stirrer speed is 60-100r / min, and the pressure inside the closed reactor is controlled in a slightly negative pressure state of -100 to -500Pa.

[0025] As a further improvement of the present invention, in the process of sulfuric acid acidolysis of the overhaul slag particles in step two, the reaction equations of fluorine, silicon, aluminum, and iron components with sulfuric acid are as follows:

[0026] Sodium fluoride reacts with sulfuric acid: 2NaF + H₂SO₄ → Na₂SO₄ + 2HF↑;

[0027] Aluminum fluoride reacts with sulfuric acid: 2AlF3 + 3H2SO4 → Al2(SO4)3 + 6HF↑;

[0028] Cryolite (Na3AlF6) reacts with sulfuric acid: 2Na3AlF6 + 3H2SO4 → 3Na2SO4 + Al2(SO4)3 + 12HF↑;

[0029] The reaction of silicon dioxide with hydrofluoric acid: SiO2 + 4HF → SiF4↑ + 2H2O;

[0030] The reaction of aluminum oxide with sulfuric acid:

[0031] The reaction of aluminum oxide with sulfuric acid: Al2O3 + 3H2SO4 → Al2(SO4)3 + 3H2O;

[0032] The reaction of iron oxides with sulfuric acid:

[0033] The reaction of iron oxide with sulfuric acid: Fe2O3 + 3H2SO4 → Fe2(SO4)3 + 3H2O.

[0034] As a further improvement of the present invention, both the gas absorption tower in step three and the ammonia absorption tower in step ten are equipped with a packed bed, which is filled with high-efficiency packing of Pall rings and Raschig rings. The circulating absorbent and ammonia absorbent are sprayed from the top of the gas absorption tower and the ammonia absorption tower respectively towards the packed bed from top to bottom. Two induced draft fans respectively introduce gas from the bottom of the gas absorption tower and the bottom of the ammonia absorption tower from bottom to top in a counter-current manner, and the gas and liquid phases contact and transfer mass on the surface of the packing.

[0035] As a further improvement of the present invention, the gas absorption tower also controls the temperature of the circulating absorption liquid at 10-20°C through a heat exchange device, and controls the liquid-to-gas ratio of the circulating absorption liquid to the gaseous product at 2-10 L / kgHF. The ammonia absorption tower also controls the temperature of the ammonia absorption liquid below 30°C through a cooling system.

[0036] As a further improvement of the present invention, a collection pool is set at the bottom of the gas absorption tower to collect a mixed solution of fluorosilicic acid and hydrofluoric acid. The circulating absorbent used by the gas absorption tower is the mixed solution of fluorosilicic acid and hydrofluoric acid in the mixed solution collection pool. When the concentration of fluorosilicic acid in the mixed solution of fluorosilicic acid and hydrofluoric acid in the mixed solution collection pool reaches 15%-20%, part of the mixed solution of fluorosilicic acid and hydrofluoric acid is sent to the ammoniation reaction pool, and water is added to the gas absorption tower to dilute the mixed solution of fluorosilicic acid and hydrofluoric acid. The diluted mixed solution of fluorosilicic acid and hydrofluoric acid continues to be used as the circulating absorbent to absorb hydrogen fluoride and silicon tetrafluoride gas. An ammonia water collection pool is set at the bottom of the ammonia absorption tower to collect ammonia water. The ammonia absorbent used by the ammonia absorption tower is ammonia water or clean water in the ammonia water collection pool. When the concentration of ammonia water in the ammonia water collection pool reaches 15%-20%, it is discharged and enters the ammonia water storage tank to provide ammonia water for the ammoniation desiliconization step.

[0037] As a further improvement of the present invention, when the solid filter cake is dried in step six, the low temperature section of the drying equipment is set to 80-120°C to remove free water; the medium temperature section of the drying equipment is set to 120-180°C to remove crystal water. This temperature range can decompose the silica-bound water remaining in the filter cake, providing a loose precursor for subsequent calcination.

[0038] As a further improvement of the present invention, in step eight, while conveying the supernatant from the sedimentation tank into the crystallization fluidized bed reactor, calcium fluoride seed crystals are added to the crystallization fluidized bed reactor. The amount of calcium fluoride seed crystals added is 1%-10% of the system mass. During the reaction process, the mixture in the crystallization fluidized bed reactor is rapidly stirred by a stirrer, and the reactor maintains a negative pressure environment of -300 to -500 Pa throughout the process, with a hydraulic retention time of 60-150 min.

[0039] A system for recovering silica and high-purity calcium fluoride from electrolytic aluminum overhaul slag includes a crushing and screening machine, a closed reactor, a gas absorption tower, an ammoniation reaction tank, an ammoniation precipitation tank, an ammonia absorption tower, a first dehydration device, a first drying device, a rotary kiln, a crystallization fluidized bed reactor, an ammonia storage tank, a blower, a booster pump, a clean water supply device, a second dehydration device, a washing tank, a second drying device, a filter, a neutralization discharge tank, and a control system. The crushing and screening machine can crush and screen the electrolytic aluminum overhaul slag. A sulfuric acid acidolysis reaction chamber is formed inside the closed reactor. The closed reactor is equipped with a heating device, a temperature sensor, and a stirring device. The heating device heats the sulfuric acid acidolysis reaction chamber inside the closed reactor, and the temperature sensor monitors the sulfuric acid... The temperature inside the acidolysis reaction chamber is monitored in real time and fed back to the control system. A stirring device agitates the mixture within the sulfuric acid acidolysis reaction chamber. A vacuum device is also installed outside the sealed reactor to evacuate the sulfuric acid acidolysis reaction chamber to a slightly negative pressure state. The exhaust port of the sealed reactor is connected to the inlet at the bottom of the gas absorption tower via a pipe. A blower sends the gas generated in the sealed reactor from bottom to top into the gas absorption tower via a pipe. The gas absorption tower contains a packed bed filled with packing material. A spray nozzle is installed at the top of the gas absorption tower to spray the circulating absorption liquid onto the packed bed from top to bottom. A mixed solution collection tank is located at the bottom of the gas absorption tower. The mixed solution collection tank collects a mixed solution of fluorosilicic acid and hydrofluoric acid formed after the dissolution of hydrogen fluoride and silicon tetrafluoride gases. The collection tank is connected via pipelines to the spray head at the top of the gas absorption tower and the inlet of the ammoniation reaction tank. Two booster pumps pump the mixed solution from the collection tank into the spray head at the top of the gas absorption tower and the ammoniation reaction tank, respectively. The outlet of the ammoniation reaction tank is connected via a pipeline to the ammoniation sedimentation tank. One pump pumps the mixed solution from the ammoniation reaction tank into the ammoniation sedimentation tank. The supernatant outlet of the ammoniation sedimentation tank is connected via a pipeline to the inlet of the crystallization fluidized bed reactor. The crystallization fluidized bed reactor is also equipped with a chemical dosing port, a seed crystal dosing port, a crystal discharge port, an exhaust port, and a water outlet. Lime solution can be... A booster pump delivers a metered amount of calcium fluoride into the crystallization fluidized bed reactor via the dosing port. Calcium fluoride seed crystals are added into the reactor via the seed crystal addition port. A crystal discharge port at the bottom of the reactor periodically discharges calcium fluoride crystals. The exhaust port at the top of the reactor is connected to the inlet at the bottom of the ammonia absorption tower via a pipe. The ammonia absorption tower contains a packed bed filled with packing material. A spray head at the top of the tower sprays ammonia absorption liquid onto the packed bed from top to bottom. An ammonia water collection tank is located at the bottom of the tower, collecting the ammonia water formed after the ammonia dissolves. This tank is connected via pipes to both the spray head at the top of the tower and the inlet of the ammonia water storage tank.Two booster pumps respectively pump ammonia water from the ammonia water collection tank into the spray head at the top of the ammonia absorption tower and into the ammonia water storage tank. The ammonia water storage tank is connected to the ammoniation reaction tank via a pipeline. One booster pump can quantitatively pump the ammonia water stored in the ammonia water storage tank into the ammoniation reaction tank to participate in the ammoniation reaction. The clean water supply device can supply clean water to the spray head at the top of the gas absorption tower and the spray head at the top of the ammonia absorption tower via pipelines. The discharge port of the closed reaction vessel can discharge the residual mixture after the overhaul slag has been acidified with sulfuric acid into the filter. The filter can perform solid-liquid separation on the residual mixture after the overhaul slag has been acidified with sulfuric acid. The filtrate from the filter can be discharged into the neutralization discharge tank for neutralization treatment via a pipeline. The sludge discharge port of the ammoniation sedimentation tank and the crystallization fluidized bed... The discharge ports at the bottom of the reactor discharge silica precipitate and calcium fluoride crystals to the first and second dehydration devices, respectively. The filter residue from the first and second dehydration devices is discharged to the first drying device and the washing tank, respectively. The first drying device dries the silica to form a loose precursor, while the washing tank washes the calcium fluoride crystals. The loose precursor and clean calcium fluoride crystals are then discharged into the rotary kiln and the second drying device, respectively. The rotary kiln calcines the silica to obtain silica, and the second drying device dries the clean calcium fluoride crystals to obtain high-purity calcium fluoride crystals. The control system controls the crushing and screening machine, the closed reactor, and the gas absorption tower.

[0040] The ammonia reaction tank, ammonia absorption tower, first plate and frame filter press, first drying equipment, rotary kiln, crystallization fluidized bed reactor, blower, booster pump, second plate and frame filter press, second drying equipment, filter and neutralization discharge tank are started and stopped.

[0041] As a further improvement of the present invention, the first dewatering device and the second dewatering device are respectively a first plate and frame filter press and a second plate and frame filter press.

[0042] The beneficial effects of this invention are as follows: This invention, through sulfuric acid acidolysis, overcomes the limitations of traditional wet treatment where fluorine, silicon, and impurities coexist in the liquid phase. Through high-temperature reaction, the fluorine and silicon components in the electrolytic aluminum overhaul slag are fully dissolved and directionally converted into recyclable gaseous products such as hydrogen fluoride and silicon tetrafluoride. This achieves efficient separation from impurities such as iron and aluminum, allowing for separate collection and preventing impurities from interfering with the purity of subsequent products at the source, laying the foundation for obtaining high-purity products. Through a combination of lime-based fluoride removal and induced crystallization, and by precisely controlling the calcium-fluorine molar ratio, seed crystal dosage, and negative pressure reaction environment, efficient and directional conversion of fluoride ions is achieved, with a stable fluoride recovery rate exceeding 95%. Simultaneously, the directional growth of calcium fluoride crystals is induced, ultimately yielding calcium fluoride products with a purity ≥92% and a particle size greater than 20 μm. This process solves the problems of low purity and small particle size of calcium fluoride in traditional processes. Furthermore, its particle size advantage improves solid-liquid separation efficiency and product processing performance, making it suitable for large-scale production and high-end applications. It offers significant improvements over traditional wet treatment methods (where fluoride recovery rates are typically below 60%), maximizing the recovery of fluoride resources from electrolytic aluminum overhaul slag and reducing the potential environmental risks of fluoride emissions. By optimizing the ammoniation silicon removal process parameters and reaction conditions, and precisely controlling the ammonia concentration, pH value, and reaction residence time, the silica precipitate generated by the reaction of fluorosilicic acid and ammonia is fully extracted. After ammoniation silicon removal, the purity of the precipitated silica can reach over 99%, meeting industrial-grade silica standards and enabling recycling. Subsequently, a lime-based directional reaction combined with induced crystallization technology promotes the directional growth of calcium fluoride crystals, ultimately yielding large-particle calcium fluoride products with a stable purity of over 92% and a particle size greater than 20 μm. Attached Figure Description

[0043] Figure 1 This is a system schematic diagram of the present invention. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Therefore, the detailed description of the embodiments of the present invention provided below is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0045] Example: A process for recovering silica and high-purity calcium fluoride from aluminum slag during overhaul, the specific steps of which are as follows:

[0046] 1. Pretreatment: The electrolytic aluminum overhaul slag is crushed and screened to remove large impurities, resulting in slag particles with a particle size of 10-50 mesh, in order to ensure the sufficiency of the subsequent sulfuric acid pyrolysis reaction;

[0047] 2. Sulfuric Acid Acidolysis: Pretreated overhaul residue particles are added to a closed reactor, along with concentrated sulfuric acid (92%-98%). The sulfuric acid ratio is set at 1:0.3-1:0.4 (solid-liquid ratio). The reaction temperature is controlled at 200-350℃. Electric heating or heat transfer oil heating can be used. The reaction time is 0.5-2 hours. A stirrer is installed inside the reactor at a speed of 60-100 r / min. A slight negative pressure is maintained inside the reactor, typically within a range of -100 to -500 Pa, to prevent the leakage of toxic gases such as HF. Through this reaction, fluorine, silicon, aluminum, iron, and other components in the overhaul residue react with sulfuric acid at high temperature, transforming into hydrogen fluoride, silicon tetrafluoride, aluminum sulfate, and ferric sulfate. The chemical equations are as follows:

[0048] Fluorides react with sulfuric acid:

[0049] Sodium fluoride reacts with sulfuric acid: 2NaF + H₂SO₄ → Na₂SO₄ + HF↑;

[0050] Aluminum fluoride reacts with sulfuric acid: 2AlF3 + 3H2SO4 → Al2(SO4)3 + 6HF↑;

[0051] Cryolite (Na3AlF6) reacts with sulfuric acid: 2Na3AlF6 + 3H2SO4 → 3Na2SO4 + Al2(SO4)3 + 12HF↑;

[0052] The reaction of silicon dioxide with hydrofluoric acid: SiO2 + 4HF → SiF4↑ + 2H2O;

[0053] The reaction of aluminum oxide with sulfuric acid:

[0054] The reaction of aluminum oxide with sulfuric acid: Al2O3 + 3H2SO4 → Al2(SO4)3 + 3H2O;

[0055] The reaction of iron oxides with sulfuric acid:

[0056] Iron oxide reacts with sulfuric acid: Fe₂O₃ + 3H₂SO₄ → Fe₂(SO₄)₃ + 3H₂O;

[0057] 3. Gas Collection: The mixed gas of hydrogen fluoride (HF) and silicon tetrafluoride (SiF4) produced by sulfuric acid pyrolysis is transported under negative pressure by an induced draft fan to a dedicated gas absorption tower for collection. The absorption tower is filled with high-efficiency packing materials such as Pall rings and Raschig rings to enhance gas-liquid contact. A spray system sprays the circulating absorbent liquid downwards from the top of the tower, while the gas is introduced counter-currently from the bottom. This ensures sufficient contact and mass transfer between the gas and liquid phases within the packing bed, allowing the hydrogen fluoride and silicon tetrafluoride in the gas phase to dissolve efficiently in the absorbent liquid. To improve absorption efficiency, the system is equipped with heat exchange equipment to precisely control the temperature of the circulating absorbent liquid at 10-20℃ and strictly regulate the liquid-to-gas ratio to 2-10 L / kgHF, ensuring a stable absorption rate of over 99% for gaseous pollutants. Silicon tetrafluoride dissolves in water to form a fluorosilicic acid solution. When the concentration of fluorosilicic acid in the collection tank at the bottom of the absorption tower reaches 15%-20%, part of the fluorosilicic acid solution is directed to the subsequent ammoniation reaction tank. At the same time, fresh water is added to the absorption tower in a timely manner to maintain the absorption capacity. The absorption mixture containing fluorosilicic acid and hydrofluoric acid is collected in the collection tank to provide raw material guarantee for the subsequent graded recovery of silicon and fluorine resources.

[0058] 4. Ammoniation for Silicon Removal and Recovery: The fluorosilicic acid solution in the collection tank is transferred to the ammoniation reaction tank, and an appropriate amount of ammonia water is added simultaneously for reaction. The ammonia water concentration is 15%-20%, the pH is controlled at 6.5-7.5, the residence time in the stirring reaction tank is 3-10 minutes, and the stirring intensity is 200W / m3-350W / m3. The effluent from the ammoniation reaction tank enters the ammoniation sedimentation tank, and the sedimentation residence time is controlled at 15-30 minutes. Through sedimentation, the silica formed in the reaction is fully separated from the water, ensuring a silicon removal rate of over 99%, while forming a precipitate that meets the requirements of silica particles.

[0059] 5. Filtration: The solid-liquid mixture in the ammoniation sedimentation tank is filtered and dewatered by a plate and frame filter press, and the filtrate enters the subsequent crystallization fluidized bed;

[0060] 6. Drying: The filter cake is dried. Low temperature range: 80-120℃, used to remove free water; medium temperature range: 120-180℃, used to remove crystal water. This temperature range can decompose the silica-bound water and bound water remaining in the filter cake, providing a loose precursor for subsequent calcination.

[0061] 7. Calcination: The precursor is calcined at 500–700℃ for 2–4 hours to ensure complete decomposition of the ammonium salt. After calcination, the silica product is obtained with a purity of over 99.0%. The specific chemical equation is as follows:

[0062] Hydrofluoric acid and ammonia react as follows: HF + NH3·H2O → NH4F + H2O;

[0063] The reaction of fluorosilicic acid and ammonia: H₂SiF₆ + 6NH₃·H₂O → SiO₂↓ + 6NH₄F + 4H₂O;

[0064] 8. Lime-induced crystallization for fluoride removal: The supernatant from the ammoniation sedimentation tank (mainly containing ammonium fluoride) is transported to the crystallization fluidized bed reactor. The reactor is started with a rapid stirring program, and a 2%-5% lime solution is added simultaneously. The calcium-fluoride molar ratio is precisely controlled at 0.45-0.50, and the hydraulic retention time is set to 60-150 min. Chemical equation:

[0065] 2NH4F+Ca(OH)2→CaF2↓+2NH3·H2O;

[0066] While conveying the supernatant from the ammoniation sedimentation tank, calcium fluoride seed crystals are added to the crystallization fluidized bed reactor at a dosage of 1%-10% of the system mass. After the reaction is completed, the effluent from the crystallization fluidized bed reactor is discharged into an intermediate water tank. After treatment by this crystallization reactor, the fluoride recovery rate can stably reach over 95%, and the reactor maintains a negative pressure environment of -300 to -500 Pa throughout the process, ensuring the safety and controllability of the reaction process.

[0067] The defluorination reactor has three core functions: First, by using rapid stirring to enhance mass transfer, it ensures that ammonium fluoride in the influent reacts fully with lime solution, directionally generating high-purity calcium fluoride precipitate, thus efficiently removing fluoride from the water. Second, by adding calcium fluoride seed crystals, it induces crystallization, guiding the calcium fluoride precipitate generated in the reaction to adhere to the seed crystal surface in a crystalline form, promoting the formation of large-particle calcium fluoride products and improving product separation performance and purity. Third, by using the alkalinity provided by lime to adjust the system environment, it promotes the conversion of ammonium ions in the water into free ammonia, creating favorable conditions for subsequent ammonia water recovery and reuse.

[0068] 9. Crystal discharge and dehydration: After the defluorination reactor has been running continuously for a period of time, the crystals gradually increase in size. The sludge solution containing large calcium fluoride particles accumulated at the bottom is periodically discharged into the crystal collection tank.

[0069] The crystal slurry collected in the crystal collection tank enters the plate and frame filter press for dewatering. After dewatering, the solid is washed and dried to obtain high-purity calcium fluoride crystals with a purity of over 92%, a silicon content of less than 0.1%, and an average particle size of over 20μm. The dewatered filtrate enters the intermediate water tank.

[0070] 10. Ammonia Recovery: Ammonia gas, a byproduct of the defluorination reaction, is introduced into the ammonia absorption tower via an induced draft fan and gas absorption pipeline. The absorption tower is equipped with packing material (Pall rings). The ammonia-rich gas and aqueous solution undergo gas-liquid contact on the surface of the packing material within the absorption tower. The ammonia in the gas is absorbed by the water to form ammonia water. After ammonia removal, the tail gas is discharged from the top of the absorption tower. To ensure absorption efficiency, a cooling system is installed to control the absorbent temperature below 30℃. To maintain the ammonia concentration, a circulating water absorption system is used. Once the ammonia concentration reaches 15%-20%, the water is discharged into an ammonia storage tank and reused as raw material for the upstream fluorosilicic acid solution desiliconization reaction. Additionally, some fresh water is added as the absorbent.

[0071] 11 Harmless treatment of pyrolysis residue: After the residual mixture of the overhaul slag is acidified with sulfuric acid, it is separated into solid and liquid. The solid is then treated to be harmless, and the liquid is neutralized with acid and alkali to meet the standards.

Claims

1. A process for recovering silica and high-purity calcium fluoride from aluminum electrolytic overhaul slag, characterized in that: Includes the following steps: Step 1: Pretreatment: Crush and screen the electrolytic aluminum overhaul slag to obtain overhaul slag particles with a particle size of 10-50 mesh; Step 2: Sulfuric acid acidolysis: The pretreated overhaul residue particles are added to a closed reactor, and concentrated sulfuric acid with a concentration of 92%-98% is added. The amount of sulfuric acid is set according to the solid-liquid ratio of 1:0.3-1:0.

4. The reaction temperature is controlled at 200-350℃. The overhaul residue particles undergo sulfuric acid acidolysis in the closed reactor. The fluorine, silicon, aluminum and iron components in the overhaul residue are converted into hydrogen fluoride, silicon tetrafluoride, aluminum sulfate and iron sulfate respectively under high temperature conditions. Step 3: Gas collection: The mixed gas of hydrogen fluoride and silicon tetrafluoride produced by sulfuric acid acidolysis is transported to the gas absorption tower by the negative pressure of the induced draft fan for collection. The hydrogen fluoride and silicon tetrafluoride gas dissolve in the circulating absorption liquid in the gas absorption tower to form a mixed solution containing fluorosilicic acid and hydrofluoric acid. Step 4: Ammoniation for Silicon Removal: The mixed solution of fluorosilicic acid and hydrofluoric acid formed at the bottom of the gas absorption tower is sent to the ammoniation reaction tank. Simultaneously, ammonia water with a concentration of 15%-20% is added to the ammoniation reaction tank to carry out the ammoniation reaction. During the reaction, the pH in the ammoniation reaction tank is controlled at 6.5-7.

5. Fluorosilicic acid reacts with ammonia water to form silicon dioxide. The chemical reaction equation is as follows: H2SiF6+6NH3·H2O→SiO2+6NH4F+4H2O; HF + NH3·H2O → NH4F + H2O; Step 5: Silica separation: The mixture in the ammoniation reaction tank is sent to the ammoniation precipitation tank. The silica formed by the ammoniation reaction is separated from the water through precipitation. Then, the solid-liquid mixture at the bottom of the ammoniation precipitation tank is dehydrated. Step Six: Drying: The solid filter residue after dehydration in Step Five is dried to form a loose precursor. Step 7: Calcination: The precursor obtained in Step 6 is calcined at 500–700℃ for 2–4 hours to completely decompose the ammonium salt. After calcination, the silica product is obtained. Step 8: Lime-induced crystallization for fluoride removal: The supernatant from the ammoniation sedimentation tank is transferred to a fluidized bed crystallization reactor, and a 2%-5% lime solution is simultaneously added to the reactor. The calcium-fluoride molar ratio is controlled between 0.45 and 0.

50. Calcium and fluoride combine to form calcium fluoride, as shown in the following chemical equation: 2NH4F+Ca(OH)2→CaF2↓+2NH3·H2O; Step 9: Crystal discharge and dehydration: The calcium fluoride crystals discharged from the crystallization fluidized bed reactor are periodically discharged. The calcium fluoride crystals discharged from the crystallization fluidized bed reactor are dehydrated, and then the dehydrated calcium fluoride crystals are washed and dried to finally obtain high-purity calcium fluoride crystals. Step 10: Ammonia Water Recovery: The gas generated during the defluorination reaction in the crystallization fluidized bed reactor is introduced into the ammonia absorption tower through the induced draft fan and gas absorption pipeline. The gas and the aqueous solution in the ammonia absorption tower come into gas-liquid contact on the surface of the packing material in the absorption tower. The ammonia in the gas is absorbed by the aqueous solution to form ammonia water. The tail gas after ammonia removal is discharged from the top of the ammonia absorption tower. Step 11: Harmless treatment of acid hydrolysis residue: After the residual mixture of the overhaul slag is acid hydrolyzed with sulfuric acid, it is separated into solid and liquid. The solid is then treated to be harmless, and the liquid is neutralized with acid and alkali to meet the standards.

2. The process for recovering silica and high-purity calcium fluoride from electrolytic aluminum overhaul slag according to claim 1, characterized in that: In step two, the closed reactor is heated by electric heating or heat transfer oil heating. The sulfuric acid acidolysis reaction time is 0.5-2 hours. During the sulfuric acid acidolysis reaction, the mixture is stirred by a stirrer at a speed of 60-100 r / min. The pressure inside the closed reactor is controlled in a slightly negative pressure state of -100 to -500 Pa.

3. The process for recovering silica and high-purity calcium fluoride from electrolytic aluminum overhaul slag according to claim 1, characterized in that: In step two, during the sulfuric acid acidolysis reaction of the overhaul slag particles, the reaction equations for fluorine, silicon, aluminum, and iron with sulfuric acid are as follows: Sodium fluoride reacts with sulfuric acid: 2NaF + H₂SO₄ → Na₂SO₄ + 2HF↑; Aluminum fluoride reacts with sulfuric acid: 2AlF3 + 3H2SO4 → Al2(SO4)3 + 6HF↑; Cryolite (Na3AlF6) reacts with sulfuric acid: 2Na3AlF6 + 3H2SO4 → 3Na2SO4 + Al2(SO4)3 + 12HF↑; The reaction of silicon dioxide with hydrofluoric acid: SiO2 + 4HF → SiF4↑ + 2H2O; The reaction of aluminum oxide with sulfuric acid: The reaction of aluminum oxide with sulfuric acid: Al2O3 + 3H2SO4 → Al2(SO4)3 + 3H2O; The reaction of iron oxides with sulfuric acid: The reaction of iron oxide with sulfuric acid: Fe2O3 + 3H2SO4 → Fe2(SO4)3 + 3H2O.

4. The process for recovering silica and high-purity calcium fluoride from electrolytic aluminum overhaul slag according to claim 1, characterized in that: Both the gas absorption tower in step three and the ammonia absorption tower in step ten are equipped with packed beds containing high-efficiency packing materials of Pall rings and Raschig rings. The circulating absorbent and ammonia absorbent are sprayed from the top of the gas absorption tower and the ammonia absorption tower downwards onto the packed beds, respectively. Two induced draft fans respectively introduce gas from the bottom of the gas absorption tower and the bottom of the ammonia absorption tower in a counter-current manner from bottom to top, and the gas and liquid phases contact and transfer mass on the surface of the packing materials.

5. The process for recovering silica and high-purity calcium fluoride from electrolytic aluminum overhaul slag according to claim 1, characterized in that: The gas absorption tower also controls the temperature of the circulating absorbent liquid at 10-20℃ through a heat exchange device, and controls the liquid-to-gas ratio of the circulating absorbent liquid to the gaseous product at 2-10L / kgHF. The ammonia absorption tower also controls the temperature of the ammonia absorbent liquid below 30℃ through a cooling system.

6. The process for recovering silica and high-purity calcium fluoride from electrolytic aluminum overhaul slag according to claim 1, characterized in that: A collection tank is set up at the bottom of the gas absorption tower to collect a mixed solution of fluorosilicic acid and hydrofluoric acid. The circulating absorbent used by the gas absorption tower is the mixed solution of fluorosilicic acid and hydrofluoric acid in the mixed solution collection tank. When the concentration of fluorosilicic acid in the mixed solution of fluorosilicic acid and hydrofluoric acid in the mixed solution collection tank reaches 15%-20%, part of the mixed solution of fluorosilicic acid and hydrofluoric acid is sent to the ammoniation reaction tank, and water is added to the gas absorption tower to dilute the mixed solution of fluorosilicic acid and hydrofluoric acid. The diluted mixed solution of fluorosilicic acid and hydrofluoric acid continues to be used as the circulating absorbent to absorb hydrogen fluoride and silicon tetrafluoride gas. An ammonia water collection tank is set up at the bottom of the ammonia absorption tower to collect ammonia water. The ammonia absorbent used by the ammonia absorption tower is ammonia water or clean water in the ammonia water collection tank. When the concentration of ammonia water in the ammonia water collection tank reaches 15%-20%, it is discharged and enters the ammonia water storage tank to provide ammonia water for the ammoniation desiliconization step.

7. The process for recovering silica and high-purity calcium fluoride from electrolytic aluminum overhaul slag according to claim 1, characterized in that: In step six, when the solid filter residue is dried, the low-temperature section of the drying equipment is set to 80-120℃ to remove free water; the medium-temperature section of the drying equipment is set to 120–180℃ to remove crystal water.

8. The process for recovering silica and high-purity calcium fluoride from electrolytic aluminum overhaul slag according to claim 1, characterized in that: In step eight, while conveying the supernatant from the sedimentation tank into the crystallization fluidized bed reactor, calcium fluoride seed crystals are added to the crystallization fluidized bed reactor. The amount of calcium fluoride seed crystals added is 1%-10% of the system mass. During the reaction process in the crystallization fluidized bed reactor, the mixture is rapidly stirred by a stirrer, and the reactor maintains a negative pressure environment of -300 to -500 Pa throughout the process, with a hydraulic retention time of 60-150 min.

9. A system for recovering silica and high-purity calcium fluoride from electrolytic aluminum overhaul slag, as described in any one of claims 1-8, characterized in that: The system includes a crushing and screening machine, a closed reactor, a gas absorption tower, an ammoniation reaction tank, an ammoniation precipitation tank, an ammonia absorption tower, a first dehydration device, a first drying device, a rotary kiln, a crystallization fluidized bed reactor, an ammonia storage tank, a blower, a booster pump, a clean water supply device, a second dehydration device, a washing tank, a second drying device, a filter, a neutralization discharge tank, and a control system. The crushing and screening machine can crush and screen the slag from electrolytic aluminum overhauls. A sulfuric acid acidolysis reaction chamber is formed inside the closed reactor. The closed reactor is equipped with a heating device, a temperature sensor, and a stirring device. The heating device heats the sulfuric acid acidolysis reaction chamber inside the closed reactor, and the temperature sensor can detect and feed back the temperature inside the sulfuric acid acidolysis reaction chamber in real time. The control system includes a stirring device to agitate the mixture within the sulfuric acid acidolysis reaction chamber. A vacuum device is also installed on the outside of the sealed reactor to evacuate the sulfuric acid acidolysis reaction chamber to a slightly negative pressure state. The exhaust port of the sealed reactor is connected to the inlet at the bottom of the gas absorption tower via a pipe. A blower sends the gas generated in the sealed reactor from bottom to top into the gas absorption tower via a pipe. The gas absorption tower contains a packed bed filled with packing material. A spray head is installed at the top of the gas absorption tower to spray circulating absorbent liquid onto the packed bed from top to bottom. A mixed solution collection tank is located at the bottom of the gas absorption tower to collect fluorinated... The mixture of hydrogen and silicon tetrafluoride gases, dissolved to form a solution of fluorosilicic acid and hydrofluoric acid, is collected in a pool connected via pipes to a spray nozzle at the top of a gas absorption tower and the inlet of an ammoniation reaction tank. Two booster pumps pump the mixture from the collection pool into the spray nozzle at the top of the gas absorption tower and the ammoniation reaction tank, respectively. The outlet of the ammoniation reaction tank is connected via a pipe to an ammoniation sedimentation tank. One pump pumps the ammoniation-treated mixture from the ammoniation reaction tank into the ammoniation sedimentation tank. The supernatant outlet of the ammoniation sedimentation tank is connected via a pipe to the inlet of a crystallization fluidized bed reactor. The crystallization fluidized bed reactor is also equipped with a dosing port, a seed crystal addition port, a crystal discharge port, an exhaust port, and a water outlet. Lime solution can be added via a booster pump. A quantitative feedstock is introduced into the crystallization fluidized bed reactor. Calcium fluoride seed crystals can be added into the reactor through the seed inlet. A discharge port at the bottom of the reactor periodically discharges calcium fluoride crystals. The exhaust port at the top of the reactor is connected to the inlet at the bottom of the ammonia absorption tower via a pipe. The ammonia absorption tower contains a packed bed filled with packing material. A spray head at the top of the tower sprays ammonia absorption liquid onto the packed bed from top to bottom. An ammonia water collection tank is located at the bottom of the tower, collecting the ammonia water formed after the ammonia dissolves. The collection tank is connected via pipes to both the spray head at the top of the tower and the inlet of the ammonia water storage tank.Two booster pumps can respectively pump ammonia water from the ammonia water collection tank into the spray head at the top of the ammonia absorption tower and into the ammonia water storage tank. The ammonia water storage tank is connected to the ammoniation reaction tank through a pipeline. One booster pump can quantitatively pump the ammonia water stored in the ammonia water storage tank into the ammoniation reaction tank to participate in the ammoniation reaction. The clean water supply device can respectively supply clean water to the spray head at the top of the gas absorption tower and the spray head at the top of the ammonia absorption tower through pipelines. The discharge port of the closed reaction vessel can discharge the residual mixture after the overhaul slag is acidified with sulfuric acid into the filter. The filter can perform solid-liquid separation on the residual mixture after the overhaul slag is acidified with sulfuric acid. The filtrate of the filter can be discharged into the neutralization discharge tank through a pipeline for neutralization treatment. The sludge discharge port of the ammoniation sedimentation tank and the crystal discharge port at the bottom of the crystallization fluidized bed reactor can respectively discharge silica precipitate and fluorine to the first dewatering equipment and the second dewatering equipment. The calcium fluoride crystals are dehydrated in the first and second dehydration devices, and the filter residues are discharged to the first drying device and the washing tank, respectively. The first drying device dries the silica to form a loose precursor, and the washing tank washes the calcium fluoride crystals. The loose precursor and the clean calcium fluoride crystals are then discharged into the rotary kiln and the second drying device, respectively. The rotary kiln calcines the silica to obtain precipitated silica, and the second drying device dries the clean calcium fluoride crystals to obtain high-purity calcium fluoride crystals. The control system controls the start and stop operation of the crushing and screening machine, the closed reaction vessel, the gas absorption tower, the ammoniation reaction tank, the ammonia absorption tower, the first plate and frame filter press, the first drying device, the rotary kiln, the crystallization fluidized bed reactor, the blower, the booster pump, the second plate and frame filter press, the second drying device, the filter, and the neutralization and discharge tank.

10. The system for recovering silica and high-purity calcium fluoride from electrolytic aluminum overhaul slag according to claim 1, characterized in that: The first dewatering device and the second dewatering device are a first plate and frame filter press and a second plate and frame filter press, respectively.

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